1. Field of the Invention
This invention relates in general to nuclear logging and in particular to a new and improved pulsed neutron logging method for determining the thermal decay time constant or tau .tau..sub.F, and correlative capture cross-section .SIGMA..sub.F of formations surrounding the borehole. In particular this invention relates to a method and apparatus for estimating the formation thermal decay constant, .tau..sub.F, more accurately than with prior methods through the simultaneous measurement of a neutron-gamma transport parameter .gamma..sub.F of a function characteristic of the diffusion of thermal neutrons in the formation. Still more particularly this invention relates to the simultaneous measurement of formation tau; .tau..sub.F ; and .gamma..sub.F, a characteristic of the diffusion of thermal neutrons in the formation and borehole tau; .tau..sub.B ; the decay constant of thermal neutrons in borehole materials; and .gamma..sub.B, a characteristic of the diffusion of thermal neutrons in the borehole materials.
2. Description of the Prior Art
This invention relates to two problems in the art of pulsed neutron logging. The first involves the measurement of formation decay rate or tau, .tau..sub.F and the simultaneous measurement of borehole decay rate, .tau..sub.B. The second relates to measurement of a characteristic of diffusion of thermal neutrons following a neutron burst not only in the formation, but also in the borehole. A discussion of background art is presented below first for the simultaneous measurement of .tau..sub.F and .tau..sub.B in borehole logging followed by a discussion of prior attempts to measure thermal neutron diffusion effects in pulsed neutron logging.